1. Field of the Invention
This invention relates to an internal reforming type molten carbonate fuel cell comprising plural unit cells, bipolar plates, and a plate-like internal reformer stacked to each other, particularly to the one of internal manifold type, and more particularly to the inside structure and arrangement of reforming catalyst in the reformer.
2. Prior Art
In a molten carbonate fuel cell (hereinafter abbreviated as MCFC), use of a reductant gas consisting mainly of hydrogen as the fuel gas or the anode active material is common practice. To obtain the fuel gas for a large scale electric power-generating system of MCFC, such a fuel gas is thought of as water gas or that obtained by reforming natural gas with water. Meanwhile, for an on-site power generation for a small demand such as a building or a unit residential area, town gas such as natural gas consisting mainly of methane is thought of. In a power generating system of small scale, however, to provide a raw-fuel-reformer which needs heating separated from the power-generator proper consisting of cell stacks resulted in a complex system with low thermal efficiency and high cost. Thus, for a MCFC for on-site power generation an internal reforming type MCFC which reforms the raw fuel supplied directly into the generator proper raised hopes for practical use and is earnestly investigated. Such internal reforming system is based on the fact that the exothermic reaction in the cells following the power generation of MCFC and Joule's heat due to ohmic resistance in and between the cells produce temperature rise of MCFC which is sufficient for the reaction reforming the gaseous raw fuel by water vapor. The endothermic property of the reforming works advantageously for the balance of thermal income and outgo. Thus, the internal reforming system having no needs for providing a reformer separate from the cell stacks is of simple and compact structure and of low cost. Moreover, the heat generation of the generator proper while being operated make the heating of the reformer unnecessary and heat efficiency of the system high, and the use of the town gas already supplied to the electric power consumer as the raw fuel results in the further reduction of power generation cost, etc. Two types of internal reforming have been proposed; direct and indirect. The direct internal reforming system, as is shown in U.S. Pat. Nos. 3,615,839, 4,182,795 et al., comprises a reforming catalyst deposited in the fuel gas chamber of the anode of the unit cell, and has the reforming and power generating reaction proceeding side by side, and attains high thermal efficiency. It, however, has the disadvantage of deterioration of the catalyst due to the molten carbonate electrolyte or electrolyte vapor through the anode since the catalyst is placed near the anode.
On the other hand, the indirect internal reforming system has the cell part reacting to generate electric power and reforming part separated from each other, wherein the raw fuel such as methane is reformed at the reforming part and the generated fuel gas is supplied to the anode.
This system, while somewhat inefficient in thermal efficiency due to the separated electric power generating part and reforming part, has the catalyst not being deteriorated rapidly. In the indirect system, the structure and the arrangement of the catalyst in the reforming apparatus is important in view of thermal balance with the cell.
The means to supply the fuel gas to the unit cell forming the cell stack of the electric power generating part is divided into two types: external manifold type and internal manifold type. The external manifold type MCFC comprises unit cells each having gateways for fuel gas of anode and for oxidant gas on both sides and these gateways are provided with outer manifolds of chest form to supply the unit cells with necessary gases. On the other hand, in the internal manifold type each of the gases is supplied to the unit cells through gas flow hole or internal manifolds provided in the frame surrounding the unit cells.
In the indirect internal reforming MCFC with outer manifold, of which a structure, for example to divide the inside of manifold into two, is shown in specifications of Japanese Patent Application Open-laying No. Sho 61-13576 or U.S. Pat. No. 5,100,743 incorporated herein by reference, the construction of the manifold is complex and reliable gas sealing is difficult. The transfer of electrolyte through the sealing material of manifold required preventive measure. Further, sometimes at the beginning of operation the height of a cell stack decreases so much that it cannot be neglected. This phenomenon arises when, at the beginning of operation, the temperature of the cell stack rises, and the electrolyte of carbonate in the mixed molding sheet consisting of the electrolyte and electrolyte holding agent melts and is impregnated in the electrolyte holding plate resulting in shrinkage of the molding sheet. Thus the outer manifold type results in more and more difficulty as the number of cells of a cell stack increases.
Meanwhile, as for the indirect internal reforming type MCFC with internal manifold there is proposed for example, a MCFC in which the unit cells are divided in two and the upper stream part thereof is provided with reforming catalyst, as is seen in the specification of Japanese Patent Application Open-laying No. Hei 3-105865 incorporated herein by reference. In such a structure, the parts of unit cells assigned to reforming have the temperature fall while at other power generating parts the temperatures rise. Thus non-uniform temperature distribution in the unit cells was a shortcoming. Another example of the indirect internal reforming MCFC of internal manifold type involves a plate-like internal reformer divided in two along its main plain surface to form two chambers, of which one is filled with reforming catalyst and the other with catalyst for combustion of not-yet-reacted fuel gas exhausted from the fuel electrode of anode. It is thought of that this internal reformer is held between two unit cells, and reformation is made using combustion heat from the catalytic combustion and reaction heat from the power generation. In this structure, however, the cell adjacent to the reforming side of the reformer is cooled and the cell adjacent to the part where the not-yet-reacted fuel gas is catalystically burnt is heated. Aside from this shortcoming, in this structure the cell stack complex has a requirement to supply air for burning not-yet-reacted fuel gas. Further, proposed is an idea to prepare plural kinds of catalyst in the direct internal reforming type MCFC to cope with various kinds of fuels, as is seen in the specification of Japanese Patent Application Open-laying No. Sho 61-34865 incorporated herein by reference. Also, the specification of Japanese Patent Application Open-laying No. Sho 63-310574 incorporated herein by reference teaches to provide reforming catalyst with less deposition of carbon at the upper stream of raw fuel than at the down stream. However, in any of these teachings no measures to make the temperature distribution in a cell uniform are found.
As described above, when indirect internal reforming type is applied to a MCFC, it is an important factor that the heat accompanied by the electric power generation in a unit cell and the heat absorbed by the reforming reaction in a reformer match each other. If the balance of these thermal inputs and outputs is not adequate, a high temperature spot or low temperature spot will arise resulting in the decrease of electric power generation efficiency.